Joining semiconductor units with bonding material

ABSTRACT

A unit includes one or more semiconductor chips. Each chip has a front surface with a plurality of contacts surrounded by a passivated surface. The passivated surface is not wettable by bonding material. The contacts have masses of bonding material thereon and the masses have a height less than the diameter of the contacts.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. patent application Ser.No. 09/523,512, filed Mar. 10, 2000, the disclosure of which is herebyincorporated by reference herein. The present application claims benefitof U.S. Provisional Patent Application No. 60/123,602, filed Mar. 10,1999, the disclosure of which is hereby incorporated by referenceherein, and also claims benefit of U.S. Provisional Patent ApplicationNo. 60/148,612, filed Aug. 12, 1999, the disclosure of which is herebyincorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] Microelectronic elements such as packaged or bare semiconductorchips, discrete components and other elements are mounted to componentssuch as rigid or flexible circuit panels by many different processes.For example, in a so-called “flip chip” or “C4” technique, a baresemiconductive chip is mounted to a circuit panel by disposing solderballs onto contact pads of the chip or circuit panel. The chip is thenmounted with its front or contact-bearing surface facing downwardly,toward the first surface of the circuit panel and with the contact padsof the chip aligned with the contact pads of the circuit panel. Theassembly is then heated to melt or “reflow” the solder whereupon theassembly is cooled leaving solid solder masses connecting each contactpad of the chip with the corresponding contact pad on the circuit panel.

[0003] As described, for example, in Multi-Chip Module Technologies andAlternatives; the Basics (Doame and Franzon, eds., 1993, pp. 450-476),considerable effort has been devoted in the art to development of theflip chip technique. Nonetheless, the flip chip technique typicallyrequires large solder balls to provide solder joints having the strengthand fatigue resistance needed to accommodate differential expansion andcontraction of the chip and the circuit panel caused by temperaturechanges during service and/or manufacture. Thus, the flip chip techniquetypically requires contact pads having large center to center spacingsor “pitch.” For these and other reasons, use of the flip chip techniquehas been limited.

[0004] As described, for example, in certain embodiments of commonlyassigned U.S. Pat. Nos. 5,148,265, 5,148,266 and 5,915,752, thedisclosures of which are hereby incorporated by reference herein,microelectronic components such as semiconductor chips may be connectedto components referred to as “interposers” such as flexible dielectricelements having terminals thereon. These connections desirably are madeso that the terminals on the connection components remain movable withrespect to the chip. A layer of a compliant material as for example, agel or an elastomer may be provided between the interposer and themicroelectronic element. These terminals can then be bonded to contactpads of a larger substrate such as a larger circuit panel. Because theterminals of the interposer remain movable with respect to the chipdifferential expansion in use does not impose substantial stresses onthe connections between the terminals and the substrate.

[0005] As described in certain preferred embodiments of U.S. Pat. No.5,518,964, the disclosure of which is hereby incorporated by referenceherein, numerous connections can be made between a microelectronicelement and a component such as an interposer by providing leads on theconnection component so that one end of each lead is permanentlyattached to the component whereas another end is releasably attached;juxtaposing the component with a microelectronic element such as a chipor wafer and bonding the tip ends of the leads to the contacts on themicroelectronic element. The microelectronic element and interposer arethen moved away from one another, typically through a predetermineddistance, so as to deform the lead towards vertically extensivedisposition. In certain other embodiments taught in the '964 patent, theleads are provided on the microelectronic element rather than on theinterposer.

[0006] In many of the processes disclosed in the aforesaid patents, anencapsulant is injected around the leads. For example, as taught incertain embodiments of the '964 patent, such an encapsulant may beinjected between the interposer and the microelectronic element duringor after the movement step so as to form a compliant layer there betweensurrounding the leads. As described, for example, in commonly assignedU.S. Pat. Nos. 5,706,174 and 5,659,952, the disclosures of which arealso incorporated by referent herein, a compliant layer may be formed byproviding a porous resilient layer such as a set of compliant standoffsbetween the connection component and the microelectronic element andconnecting the contact pads of the microelectronic element to leads orother conductive features on the connection component. Typically afterthese connections have been made, a flowable material such as a curableencapsulant is introduced into the porous layer as, for example, betweenthe compliant standoffs and cured so that the flowable material and theoriginal porous layer form a composite complaint layer. The flowablematerial desirably also encapsulates the electrical connections.

[0007] U.S. Pat. No. 5,798,286, the disclosure of which is alsoincorporated by reference herein describes, in certain embodiments,techniques wherein a plurality of individual semiconductor chips orother separately formed microelectronic elements are assembled to asingle connection component or interposer, as, for example, a dielectricsheet having leads thereon with releasable tip ends as discussed abovewith respect to the '964 patent. In certain disclosed embodiments of the'286 patent, the individual chips are held in a heated chuck and engagedwith the tip ends of the leads so that bonding materials such aseutectic bonding alloys, solders or the like carried on the chip or onthe leads are activated to form bonds between the chip and the leads. Asalso disclosed in certain embodiments of the '286 patent, and in certainembodiments of commonly assigned U.S. Pat. No. 5,766,987, the disclosureof which is hereby incorporated by reference herein, a covering layermay be provided over the rear surfaces of the chips so as to protect therear surfaces of the chips from encapsulant contamination during theprocess.

[0008] The processes provided by the aforementioned patents providesubstantial improvements in the art. Nonetheless, even furtherimprovement would be desirable. For example, the heat applied during abonding process may cause polymeric layers incorporated in componentssuch as interposers to expand, making it more difficult to achieveprecise alignment between the conductive features of the microelectronicelements and the conductive features of the connection component.Certain bonding materials, particularly solders, require fluxes at theconnections to make a sound joint. These fluxes can cause problems inservice unless they are removed by a cleaning process, which adds costto the process. Also, many of the techniques commonly used in dispensingsolder onto parts to be assembled do not lend themselves to extremelyfine-pitch assembly work. Therefore, further improvements in bondingprocesses and in processes for applying bonding materials such assolders would be desirable.

SUMMARY OF THE INVENTION

[0009] One aspect of the present invention provides improved processesfor making electrically conductive bonds between microelectroniccomponents. In preferred processes according to this aspect of theinvention, bonds such as solder joints are formed between contacts on afirst microelectronic element such as a semiconductor chip or wafer andconductive elements of a second microelectronic element such as aconnection component, by momentarily heating the first microelectronicelement so as to activate a bonding material and then allowing the firstmicroelectronic element to cool, leaving the contacts on the firstmicroelectronic element bonded to conductive features on the secondelement or connection component. In preferred processes according tothis aspect of the invention, the second element or connection componentis maintained at an average temperature below the average temperature ofthe first microelectronic element during the momentary heating step.Stated another way, the temporary heating steps is performed so that thebonds are formed while the first microelectronic element is at a highertemperature than the second element or connection component. Themicroelectronic elements are not in thermal equilibrium with oneanother. Most preferably, where the second element or connectioncomponent includes a body formed wholly or partially from a polymer, thesecond element is maintained at an average temperature below the glasstransition temperature of the polymer. This markedly reduces dimensionalchanges in the polymeric body.

[0010] Most desirably, the temporary heating and cooling steps areperformed while the elements are maintained under a vacuum, i.e., asubatmospheric total pressure from most typically about 10-50 milliTorr.The vacuum inhibits heat transfer between the microelectronic elements.The first microelectronic element desirably is heated by directingradiant energy onto it. For example, where the first microelectronicelement is a wafer or chip having a back surface facing away from thesecond microelectronic element or connection component, radiant energysuch as infrared radiation in a wavelength band absorbed by the materialof the chip or wafer can be directed unto the back surface. This heatsthe wafer rapidly.

[0011] A further aspect of the present invention provides methods ofapplying bonding material such as solder to form bonding material massesor “bumps” on a microelectronic element such as a chip or a wafer. Inpreferred methods according this aspect of the invention, the elementhas a non-wettable surface surrounding the contacts, i.e., a surfacewhich cannot be wetted by the bonding material. The bonding material isapplied in liquid form onto the element, such as on the contact bearingfront face of the chip or wafer, so that the surface as a whole,including both the contacts and the non-wettable surface, is exposed tothe liquid bonding material. The liquid bonding material adheres only tothe contacts. The liquid bonding material forms small droplets havingthe natural shape of a meniscus due to surface tension. As furtherexplained below, these droplets may have a very small volume. Forexample, the volume of such droplets may be substantially smaller thanthe volume of a spherical solder ball having a diameter equal to thediameter of the contact. Small solder masses, such as themeniscus-shaped solder masses, greatly reduce the probability ofshorting between adjacent contacts during a bonding operation. They areparticularly valuable with small contacts as, for example, contactshaving diameters less than about 100 microns and/or contacts spaced atcenter to center distances or “pitch” less than about 200 microns.

[0012] A further aspect of the present invention provides a method ofpreparing a microelectronic element including the step of providing amicroelectronic element having a plurality of contacts arranged in oneor more rows, the contacts in each row being spaced apart from oneanother in a row direction. The method further includes applying aliquifiable bonding material as, for example, a solder paste to theelement in elongated strips extending across the contacts transverse tothe row direction. The lengthwise or elongation direction of eachbonding material strip is transverse to the row direction. Desirably,the strips have widthwise dimensions in the row direction less than thedimensions of the contacts in the row direction. The method desirablyincludes the further step of bringing the bonding material to a liquidcondition as, for example, by heating the bonding material. When thebonding material is in a liquid condition, it wets the contactspreferentially and does not substantially wet a surface of themicroelectronic element surrounding the contact. Thus, surface tensionin the liquid bonding material forms the bonding material in each stripinto a mass covering the contact crossed by such strip. Applying thebonding material in the form of elongated strips transverse to the rowdirection provides spaces between the edges of adjacent bonding materialstrips substantially greater than the spaces between adjacent edges ofthe contacts in the row. Also, the spaces between edges of adjacentbonding material strips is greater than the spacing which would existbetween adjacent edges of bonding material masses if the bondingmaterial were applied in the form of a circular or square blob. Thislarge spacing between bonding material masses prior to liquefactionallows application of the bonding material by techniques having limitedprecision such as stenciling without causing adjacent boning materialmasses to touch one another. When the bonding masses are liquefied, theywill be effectively confined to the areas encompassed by the individualcontacts and will remain out of its contract with one another. Thistechnique allows application of solder pastes and other liquifiablebonding materials on contact space at a fine pitch. It also allowsformation of relatively small, meniscus-shaped solder masses asdiscussed above. In a variant of this approach, the liquifiable bondingmaterial is applied in masses which touch individual contacts in the rowbut which have their centers of area offset from the row centerline.Alternate masses have centers offset in opposite directions from the rowcenterline. The masses may not include elongated strips. Here again,there are relatively large spaces between adjacent masses.

[0013] A further aspect of the present invention provides methods ofmaking microelectronic assemblies. A method in accordance with thisaspect of the invention desirably includes the steps of providing one ormore microelectronic elements and one or more components with adeformable barrier such as a flexible sheet. These are provided so thatconductive features of the microelectronic elements and componentsconfront one another in a working space at least partially bounded bythe deformable barrier. The method according to this aspect of theinvention further includes the step of maintaining the partial pressureof oxygen below the partial pressure of oxygen providing in normalatmospheric air at atmospheric pressure, i.e., below about 160 Torr, andmaintaining the working space under a total absolute pressure lower thanthe total absolute pressure prevailing outside of the working space. Apressure differential on the barrier urges the barrier into the workingspace and the barrier urges conductive features on the one or moremicroelectronic elements into engagement with conductive features on theone or more components. A bonding material such as solder is activatedat the engaged conductive features. This activation occurs at leastpartially while the atmosphere within the working spaces maintained atthe aforesaid low partial pressure of oxygen. Most preferably, the totalabsolute pressure within the working space is below normal atmosphericpressure, and the activation step is performed while ambient atmosphericconditions prevail outside the working space. The desired low partialpressure of oxygen can be achieved simply by withdrawing air from theworking space to lower the total pressure or, more preferably, by firstflushing the working space with a non-oxidizing gas and then bringingthe working space to the desired subatmospheric total pressure. The stepof activating a bonding material desirably includes momentarily heatingthe bonding material and the conductive features as, for example, byapplying radiant energy.

[0014] As further discussed below, connection of the bonds under a lowoxygen partial pressure facilitates bonding, and particularly solderbonding, without flux. Because the deformable barrier in conjunctionwith other components, encloses the working space, the process can beperformed under normal ambient atmospheric conditions, outside of avacuum or pressure chamber. For example, where the components andmicroelectronic elements are heated by application of radiant energy,this step can be performed outside of a vacuum chamber. Moreover, themechanical action of the deformable barrier or film urging theconductive features together helps to assure reliable engagement andbonding even where the parts carrying the conductive features are out ofplane or of an uneven thickness. The deformable barrier may be a film orother deformable element formed separately from the microelectronicelements and components. Alternatively, the component or components mayserve as part of all of the deformable barrier. For example, where thecomponent or components includes a flexible, sheet-like dielectricelement carrying conductive features, the dielectric element itself mayserve as the deformable barrier.

[0015] A related aspect of the invention provides additional methods ofmaking microelectronic assemblies. A method according to this aspect ofthe invention includes the step of providing one or more microelectronicelements in a working space between a flexible film and one or morecomponents so that a front face of each microelectronic element withconductive features exposed thereon confronts a front face of acomponent having conductive features exposed thereon such front face andso that a rear surface of each microelectronic element faces upwardlyaway from the one or more components and toward the film. The conductivefeatures of the elements and components are aligned with one another.This aspect of the invention further includes the step of maintainingthe working space under an absolute pressure less than the absolutepressure prevailing outside of the working space, so that the film urgesthe microelectronic elements downwardly against the components and thusbiases the conductive features into engagement with one another. Whilethis pressure differential is present radiant energy is directed intothe working space, preferably through the film and onto the one or moremicroelectronic elements to thereby momentarily heat the engagedconductive features and activate a bonding material on the engagedconductive features so as to bond these features to one another. Hereagain, the step of maintaining the working space desirably includesmaintaining the working space at a subatmospheric total pressure, andthe step of directing radiant desirably is performed while the exteriorof the film is exposed to ambient atmospheric pressure. Here again,because radiant energy exposure operation and hence bonding can beperformed outside a vacuum chamber, the process can be performed at lowcost, using simple equipment. Moreover, the components andmicroelectronic elements can be cooled rapidly when exposure to radiantenergy is terminated.

[0016] Most preferably, the step of providing the microelectronicelements and components includes providing the one or more components onan upper surface of a fixture and the step of maintaining the workingspace at subatmospheric pressure includes sealing a peripheral region ofthe film to the one or more components or to the fixture and withdrawinggas from the working space.

[0017] The flexible film desirably is sealingly connected to the rearsurface or surfaces of the one or more microelectronic elements. Themethod may further include the step of injecting a curable materialbetween the flexible film and the one or more components so as toencapsulate the conductive features after bonding. Thus, the sameflexible film which bounds the working space during the bonding stepalso serves to protect the rear surfaces of the microelectronic elementsduring the encapsulant injection step. The conductive features on themicroelectronic elements, on the components or both, may include leads,and the method may further include the step of moving themicroelectronic elements and components away from one another to therebydeform the leads. This step may occur, for example, concomitantly withencapsulant injection. Particularly preferred methods according to thisaspect of the invention include the step of temporarily securing themicroelectronic elements to the one or more components prior toactivation of the bonding material as, for example, when themicroelectronic elements are first placed onto the one or morecomponents. The temporary attachment is detached during or after thebonding step as, for example, before or during the step of moving thecomponents and microelectronic elements away from one another.

[0018] A further related aspect of the invention provides methods ofmaking microelectronic assemblies including the step of temporarilysecuring one or more elements in place on one or more components byproviding temporary securements extending between the microelectronicelements and components. The temporary connecting elements desirablyadhere to the microelectronic elements and to the components. The methodaccording to this aspect of the invention desirably further includes thestep of connecting conductive features of the one or moremicroelectronic elements to conductive features of the one or morecomponents and releasing the temporary securements during or after theconnecting step. Typically, the conductive features are exposed on frontfaces of the one or more microelectronic elements and on first surfacesof the one or more components and the temporary securing step isperformed so that the front faces of the microelectronic element orelements confront the first surfaces of the one or more components, sothat the conductive features are aligned with one another and so thatthe temporary connecting elements extend between the confrontingsurfaces of the microelectronic elements and components. According tothis aspect of the invention, microelectronic elements such as chips canbe aligned with the components and bonded to under moderate conditions,desirably at normal ambient room temperature. This simplifies the taskof placing the microelectronic elements accurately. Once themicroelectronic elements are disposed in the correct positions relativeto the components, flexible film as discussed above can be placed overthe rear surfaces of the microelectronic elements without moving them,inasmuch the microelectronic elements are held in place by the temporarysecurement.

[0019] In some embodiments the temporary securements can be released byapplication of heat. For example, where an encapsulant is applied aroundthe conductive features and brought to an elevated temperature, thetemperature used to cure the encapsulant may be sufficient to releasethe temporary securement. Alternatively or additionally, momentaryheating applied to bond the conductive features to one another may serveto release the temporary securements from adhesion or to destroy thetemporary securements. The temporary securements typically includeorganic materials such as thermoplastics or other polymers which can bedegraded by application of heat.

[0020] A further aspect of the invention provides components for use inmaking microelectronic assemblies, as, for example, in the processesdiscussed above. A component according to this aspect of the inventionincludes a dielectric body having conductive features exposed at a firstsurface and also includes one or more “dummy pads” exposed at the frontsurface. The dummy pads may be formed from conductive or dielectricmaterial. The dummy pads have release portions which are releasablyconnected to the dielectric body. One or more masses of temporarysecurement material are provided on the release portions of the dummypads. The temporary securement material may be an adhesive material orother material adapted to bond to a microelectronic element when themicroelectronic element is disposed over the front surface of thedielectric body. As further discussed below, the dielectric body mayhave holes extending through it to the dummy pads and the releaseportions of the dummy pads may be disposed in registration with theholes. Anchor portions of the dummy pads extend beyond the holes andonto the dielectric body. In use, the release portions of the dummy padsmay break away from the anchor portions of the dummy pads.

[0021] These and other objects, features and advantages of the presentinvention will be more readily apparent from the detailed description ofthe preferred embodiments set forth below, taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a diagrammatic sectional view depicting amicroelectronic element during one stage of a process in accordance withone embodiment of the invention.

[0023]FIG. 2 is a fragmentary, diagrammatic sectional view on anenlarged scale depicting a portion of the component of FIG. 1.

[0024]FIGS. 3 and 4 are diagrammatic sectional views depicting thecomponent of FIGS. 1 and 2 during later stages of the same process.

[0025]FIG. 5 is a fragmentary, diagrammatic sectional view depictingcomponents during a process in accordance with a further embodiment ofthe invention.

[0026]FIG. 6 is a fragmentary, diagrammatic plan view depicting portionsof a component during a process in accordance with a further embodimentof the invention.

[0027]FIGS. 7, 8A and 8B are views similar to FIG. 6 but depictingcomponents during processes in accordance with other embodiments of theinvention.

[0028]FIG. 9 is a diagrammatic elevational view depicting portions ofcomponents during a process in accordance with a further embodiment ofthe invention.

[0029]FIG. 10 is a fragmentary, diagrammatic view on an enlarged scaledepicting the area indicated in FIG. 8.

[0030]FIGS. 11, 12, 13, 14 and 15 are further diagrammatic elevationalviews depicting the components of FIGS. 9-10 during progressively laterstages of the process.

[0031]FIG. 16 is a fragmentary, diagrammatic view on an enlarged scaledepicting the area indicated in FIG. 15.

[0032]FIG. 17 is a diagrammatic sectional view depicting components in aprocess according to a further embodiment of the invention.

[0033]FIGS. 18 and 19 are views similar to FIG. 17 but depictingcomponents in processes according to still further embodiments of theinvention.

[0034]FIGS. 20 and 21 are fragmentary diagrammatic sectional viewsdepicting components during processes according to still otherembodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] A process according to one illustrative embodiment of theinvention begins with a semiconductor wafer 10 (FIG. 1) incorporating alarge number of semiconductor chips in a single unit. Wafer 10 has arear surface 12 and a front surface 14 with electrically conductivecontacts 16 thereon. Contacts 16 are formed with a solder-wettable metalat their surfaces. For example, nickel, gold copper or combinationsthereof can be provided at the contact surfaces. One typical arrangementprovides aluminum contacts with nickel over the aluminum and gold overthe nickel. Another typical arrangement provides chromium over analuminum contact, copper over the chromium and gold over the copper. Thefront surface 14 also has a passivation layer 18 covering those areasnot occupied by contacts 16. The passivation layer may be formed from aconventional silicon oxide, silicon nitride or other inorganic compound.Alternatively or additionally, the passivation layer may includepolymeric materials such as polyimide. The thickness of the contacts andthe passivation layer in the vertical directions, normal to the frontand rear surfaces of the wafer are greatly exaggerated in FIG. 1 forclarity of illustration.

[0036] Solder masses 20 are applied on contacts 16 by a wave solderingoperation. As diagrammatically indicated in FIG. 1, the front surface 14of wafer 10 faces down towards the top of a tank of molten solder 22. Awave is created in the solder by conventional equipment. Either thecrest of the wave, the wafer or both move, so that the crest of the wavemoves relative to the wafer, thereby contacting the entire front surfaceof the wafer. The molten solder contacts both the non-wettablepassivation layer 18 and the wettable contacts 16 but adheres only tothe contacts, thereby forming masses 20. As diagrammatically illustratedin FIG. 2, masses 20 are not spherical but instead have a flattened,dome-like shape caused by surface tension effects. Stated another way,each mass is in the shape of a meniscus. The exact shape of the meniscuswill depend on factors such as the surface energies of the moltensolder, the metal of the contacts 16 and the surrounding passivationlayer. However, the outside diameter d of each solder mass 20 typicallyis equal to the diameter of the exposed area of the contact (the areanot covered by passivation layer 18) whereas the height or verticaldimension h of each solder mass in the vertical direction, perpendicularto the front surface of the wafer typically is substantially smallerthan the diameter of the mass. Each solder mass has a volumesubstantially smaller than the volume of a conventional spherical solderball of equal diameter. Stated another way, the volume of each soldermass is less than πd³/6. In the use of solder masses formed on square orother non-round contacts, the diameter d can be taken as the meanhorizontal dimension of the mass. The solder in the masses solidifiesrapidly, by heat transfer to the wafer. The wafer may be cooled duringapplication of the solder by a coolant or cold plate in contact withrear surface 12 to facilitate cooling of the solder and to limit thetemperature attained within the wafer.

[0037] In the next stage of the process, wafer 10 is juxtaposed with asecond microelectronic element or connection component 30. In theparticular embodiment illustrated, component 30 includes flat,sheet-like body 32 having a top surface 34 and a bottom surface 36. Inthe illustration, body 32 is shown as a simple sheet of a polymericmaterial without any internal reinforcement or internal components.However, body 30 may include internal metallic components such aselectrically conductive ground planes or traces, and may further includemetallic or other components on its surfaces, such as electricallyconductive traces extending along the surfaces. Body 32 is bonded to arigid frame 38 defining a central opening 40 so that the body 32 extendsacross the central opening and so that the frame holds body 32 taut. Theuse of a frame to reinforce a flexible component such as a sheet-likeelement during processing and assembly operations is described ingreater detail in U.S. Pat. No. 5,518,964 and also in co-pending,commonly assigned U.S. patent application Ser. Nos. 09/173,797, filedOct. 16, 1998 and 09/174,074, filed Oct. 16, 1998, the disclosures ofwhich are hereby incorporated by reference herein.

[0038] The second microelectronic element or connection component haselectrically conductive bonding features 42 overlying top surface 34. Inthe particular embodiment illustrated, each such bonding feature isformed as a tip end of a lead 44. Each lead 44 has terminal end 46fixedly connected to body 32. For example, the terminal ends 46 aredepicted in FIG. 3 as being attached to vias 48 extending through body32 and forming terminals exposed at the second or bottom surface 36 ofthe connection component body 32. However, the terminal ends 46 may beanchored to the body in other ways. The tip ends or conductiveconnecting features 42 are connected to the top surface 34 of body 32 bysmall connecting elements 50. Connecting elements 50 havecross-sectional dimensions, in horizontal directions parallel to the topsurface 34, smaller than the corresponding dimensions of conductiveconnecting features 42. These small connecting features serve toreleasably attach the tip ends or conductive connecting features 42 tothe top surface 34. The use of small features to form a releasableattachment between an end of a lead and a component is described ingreater detail in the aforementioned '964 patent and also in co-pendingcommonly assigned U.S. patent application Ser. Nos. 09/020,750 filedFeb. 9, 1998; 09/195,371, filed Nov. 18, 1998, and in PCT InternationalApplication PCT/US99/02748, the disclosures of which are alsoincorporated by reference herein. As disclosed in the aforementionedpatent and patent applications, the connecting elements may be formedfrom the metal used to form the electrically conductive components orelse may be formed from the polymer defining top surface 34. Theconnecting elements may adhere weakly to the top surface or may bebreakable. With either arrangement, the conductive features or tip ends42 of the leads are releasably-held to the top surface of connectioncomponent body 32.

[0039] The juxtaposed first microelectronic element or wafer 10 andsecond element or connection component 30 are placed in a vacuum chamber(not shown). The bottom surface 36 of the connection component body,facing away from the first element or wafer 10 is engaged with atemperature stabilizing block 52. Block 52 in turn is maintained at apre-selected temperature, desirably below the glass transitiontemperature of the polymer in body 32 by appropriate temperature controldevices such as a device for circulating a control-temperature fluidthrough the block or other conventional temperature control devices. Forexample, where the polymer is a polyimide having a glass transitiontemperature of about 300° C., the temperature of stabilizing element orblock 52 desirably is between about room temperature (20° C.) and about200° C., and should be below the glass transition temperature of thepolymer. The connection component or second element 30 and the wafer orfirst element 10 come to thermal equilibrium at a temperature close tothe temperature of stabilizing element 52. This equilibrium temperatureis slightly below the solidus temperature of the solder in masses 20.The gas within the chamber is brought to a vacuum, i.e., asubatmospheric total pressure of about 1-50 milliTorr, more typically10-50 milliTorr. The partial pressure of oxygen is less than the totalpressure. To reduce the partial pressure of oxygen still further, thechamber may be flushed with a non-oxidizing gas before the vacuum isapplied. Under these conditions, oxides and other metal compounds formedon the surfaces of solder masses 20 decomposed, leaving clean metalsurfaces. The use of a high vacuum to cause decomposition of oxides onsolder masses is described, for example, in an article entitled FluxFree Process for Placement and Attach of Solder Balls to Wafers, FlipChips and All BGA Packages, by Richard Ramos, and the data sheetentitled Controlled Atmosphere Large Format Vacuum Furnace, modelPF-2400, the disclosures of which are also incorporated by referenceherein.

[0040] While the components are maintained under vacuum conditions, thefirst microelectronic element or wafer 10 is momentarily heated byapplying radiant energy to the rear surface 12, as schematicallyindicated by arrows 54 in FIG. 3. Desirably, the radiant energy is in aband of wavelengths which is strongly absorbed by at least some of thecomponents in the first element or wafer so that little of the radiantenergy passes through the wafer to the second element or connectioncomponent 30. For example, the radiant energy may be applied by one ormore halogen lamps or infrared lamps, typically disposed about 0-1 cmfrom the rear surface of the wafer. In one typical process, four lamps,each energized at 300 watts, are used to heat a 5 inch (12.7 cm)diameter wafer, so as to provide a power density of about 10 watts/cm².The lamps are energized at this power level for 120-180 sec. Greater orlesser power levels and heating times can be used. Alternatively thelamp or lamps may be continually energized and a shutter may disposedbetween the lamp and the rear surface of the wafer. The shutter may beopened, typically for about 120-180 sec and then rapidly closed. Thepulse of applied infrared energy momentarily raises the temperature ofthe first element or wafer 10 to a peak temperature above the liquidustemperature of the solder in masses 20. The peak temperature attained isreasonably uniform throughout the wafer, inasmuch as the radiation isapplied substantially uniformly over the rear surface of the wafer andthe wafer is a reasonably good conductor of heat.

[0041] Heat conducted from the wafer and through contacts 16 and thesurrounding portions of the wafer into solder masses 20 raises thetemperature of the solder masses above their liquidus temperature. Thisalso raises the temperature of the conductive features or tip ends 42 onthe second element to a temperature at least sufficient to provide goodinterdifussion between the solder and the metal of the conductivefeature, i.e., a temperature sufficient to cause metallurgical bondingat the solder with the conductive features of the second element 30.

[0042] After the wafer 10 has been brought to its peak temperature, thetemperature of the wafer rapidly declines due to heat transfer from thewafer. Such heat transfer can occur, for example, by radiation from thewafer to the surrounding surfaces of the vacuum chamber. As thetemperature of the wafer momentarily rises and then falls back againtowards its equilibrium temperature, the average temperature of thesecond element or connection component body 32 remains close to thetemperature of the stabilizing element 52 and hence remains below theglass transition temperature of the polymer. That is, because there isvery good heat transfer between the connection component body and thestabilizing element 52, and very poor heat transfer between the firstelement 10 and the body 32 of second element 30, the average temperatureof the connection component remains substantially lower than the averagetemperature of the first element during the momentary rise and fall inthe first element temperature. Because the average temperature ofcomponent body 32 remains below the glass transition temperature of thepolymer, it does not undergo unpredictable dimensional changes, andhence the conductive features or tip ends 42 of the leads remain in aknown, predictable positions. Although the momentary heating of thewafer may cause the wafer to expand slightly, this effect is readilypredictable and relatively small compared to the dimensional changeswhich can occur in a polymeric element heated to above its glasstransition temperature. Such predictable expansion can be compensatedfor in the design of the mating elements. When the wafer has cooled, thesolder solidifies and forms bonds with the conductive features or tipends 42 of the leads.

[0043] As pointed out above, there must be enough heat transfer into andthrough the solder masses so as to heat the solder masses 22 to theirliquidus temperature and also heat the adjacent surfaces of theconductive features or tip ends 42 to a temperatures close to thetemperature of the solder. However, heat transfer through the soldermasses and other pathways should not raise the average temperature ofthe connection component or second element above the glass transitiontemperature of the polymer incorporated in that component. The vacuumenvironment greatly facilitates this operation, inasmuch as the vacuumsubstantially prevents convective heat transfer from the wafer 10 to thebody 32 of the connection component or second element 30. Also, becauseeach of the conductive elements 42 is connected to the body 32 by arelatively small connecting element 50, conductive heat transfer betweenthe electrically conductive features or tip ends 42 and the body 32 islimited. Conversely, the relatively low height of solder masses 20 (FIG.2) facilitates rapid heat transfer from the wafer through the soldermasses to the surfaces of the conductive features or tip ends 42. All ofthese factors, taken together make it relatively easy to bring thesolder masses and conductive features to a relatively high temperaturewhile maintaining the body 32 at a relatively low temperature. It shouldbe appreciated that some portions of body 32 immediately below theconductive features 42 may reach temperatures above the glass transitiontemperature of the polymer. However, the surrounding areas of thepolymer remain dimensionally stable and hence localized heating in theseregions does not materially affect dimensional stability of the part asa whole.

[0044] After forming the bonds, the elements 10 and 30 are moved awayfrom one another so as to deform leads 44 towards a vertically extensivedisposition 44′ as seen in FIG. 4. This moves the conductive features ortip ends 42 away from the top surface 34 of the second element, andeither breaks connecting elements 50 or peels them away from theconductive feature or the body surface. In the vertically extensivecondition 44′, the leads are flexible and allow movement of tip ends 42relative to terminal ends 46 and relative to body 32. A curable liquidmaterial such as a dielectric encapsulant 60 is injected between theelements during or after movement of the elements away from one anotherand cured to form a compliant dielectric layer. These steps can beaccomplished in substantially the same way as described in theaforementioned patents and applications. The resulting product may becut into individual units, each including one or more wafers andassociated portions of connection component 30, by cutting the wafer andconnection component along predetermined scribe lines 62. As describedin greater detail in the aforementioned patents and publications, theresulting packaged semiconductor chip can be mounted to a circuit boardor other device. The terminals 48 may be bonded to the circuit board.Differences in thermal expansion and other factors may cause movement ofterminals 48 with respect to contact 16 on the chip in the service orduring manufacture as, for example, during the heating and coolingassociated with mounting to a circuit board. However, such differentialmovement is taken up by the flexible leads 44′. It does not causeappreciable stress on the solder masses 20. Stated another way,provision of a flexible or deformable connection to the solder massesallows the use of small solder masses. There is no need to provide asolder mass of sufficient dimensions to take up relative movement bydeformation of the solder mass itself. Thus, the combination of adeformable connection and a small solder mass provides unique benefits.The small solder masses facilitate the use of fine-pitch contacts,whereas the flexible connections to the solder masses help the smallsolder masses to survive in service where such solder masses wouldotherwise be inadequate.

[0045] Numerous variations and combinations of the features discussedabove can be utilized. For example, the designations “top” and “bottom”are used merely for convenience of reference. The entire arrangement canbe provided in any position relative to the normal, gravitational frameof reference, so that, for example, bottom surface 36 may face upwardlyin the gravitational frame of reference. Also, the arrangement of leadsand conductive features can be reversed so that the wafer or chip 110(FIG. 5) is provided with leads 144 having fixed or terminal ends 146permanently bonded to contacts 116 of the wafer and having tip orreleasable ends 142 releasably connected to the surface of the wafer, asfor example, by a low adhesion bond between the tip ends 142 and thepassivation layer 118. Further, the solder masses may be provided on theconnection component, rather than on the wafer. In FIG. 5, theconnection component 130 has fixed conductive features 143 provided withsolder masses 120 in the manner discussed above with reference to thewafer.

[0046] Also, other ways of momentarily heating a component can be usedapart from radiant energy. For example, the rear surface of the wafermay be exposed momentarily to a hot heat transfer fluid such as a liquidor a hot gas. If desired, such exposure can be followed by promptexposure to a cold heat transfer fluid. Appropriate precautions shouldbe taken to avoid contaminating the components with any such heattransfer fluids. Also, where radiant energy is employed, radiant energymay be directed onto the first element or wafer through the secondelement, provided that the radiant energy is not strongly absorbed bythe second element. This arrangement is less preferred inasmuch as itimposes constraints on the design of the second element. For example,this arrangement cannot be used readily where the second elementcontains a large amount of metal in the form of ground planes or thelike. Further, although the process has been described with reference toforming bonds between wafers and connection components, the same processcan be used with other microelectronic elements. Merely by way ofexample, the process can be used to form connections between two wafers;between two chips; or between two connection components.

[0047] As illustrated in FIG. 6, a microelectronic element, such as awafer, connection component or the like may have numerous contacts 216arranged in rows. Each row of contacts a whole extends in a rowdirection, and the contacts are spaced apart from one another in the rowdirection (to the left and right), along line R as seen in FIG. 6).Although only one row is shown in FIG. 6, the actual component mayinclude one row or plural rows having the same row directions ordifferent row directions. The contacts are surrounded by a surface 218which is substantially non-wettable by the molten bonding material.Portions of this surface extend between adjacent contacts 216. Surface218 is substantially less wettable by the molten bonding material thanthe surfaces of the contacts 216. This surface 218 may be a typicalpassivation-layer surface such as a layer of silicon oxide, siliconnitride or a polymeric material such as polyimide, whereas the surfacesof contacts 216 may be formed from a solder-wettable material such asgold, copper or combinations thereof. It is desirable to make the pitchor center-to-center distance as small as practicable consistent withgood bonding and good reliability during service.

[0048] In a method according to a further embodiment of the invention,solder is applied to the contacts by stenciling, silk-screening orotherwise depositing elongated strips 220 of solder paste. Each strip220 extends across a contact 216. The direction of elongation of eachindividual strip is transverse to the row direction. Each strip has awidth-wise dimension w_(s) smaller than the dimension w_(p) of the padin the row direction. Thus, the edge to edge spacing S_(s) betweenadjacent strips 220 is greater than the edge to edge spacing S_(c)between adjacent contact pads. Each strip 220 desirably extends acrossthe associated pad 216 near the center of the contact.

[0049] Solder pastes are mixtures of a paste constituent, which mayinclude a flux, together with finely powdered metal solder. The solderpaste typically also include a solvent with or without additives such asthickening agents for control of viscosity. A solder paste may have aliquid or pasty consistency when first applied and may set to a firmerconsistency by evaporation of the solvent after application. Solderpastes typically are applied by stenciling, silk screening orapplication of through a fine dispensing needle. Upon heating, thesolder in the case melts and the flux and other outraging componentsreact or evaporate, leaving a liquid metallic solder. Conventionalsolder paste application processes are discussed, for example, inSolberg, Design Guidelines for Surface Mount and Fine-Pitch Technology,Second Edition, pp. 218-226, McGraw-Hill, 1996, the disclosure of whichis hereby incorporated by reference herein. Solder paste typically isapplied with the component at about room temperature, i.e., attemperatures well below the melting point of the solder. The solderpaste, with its metal particles dispersed in an organic phase, wets thesurface of layer 218 as well as the surface contacts 216 and henceremains in the as-deposited strip-like form extending over both thecontacts and the adjacent portions of surface 218.

[0050] In the next stage of the process, the component 220, with thesolder paste strips thereon, is heated to a temperature above thesolidus temperature of the metal in the solder paste and desirably abovethe liquidus temperature of the metal. The organic components evaporateor separate from the molten metal, leaving a mass of molten metalcorresponding to each paste strip 220. The molten metal in each strippreferentially wets the surface of the associated contacts 216 and flowsto form a mass atop the contact. Stated another way, surface tension andinterfacial tension reshape the molten solder from the strip shape ofthe originally-applied paste to the shape 220′ corresponding to theshape of the associated contact 216.

[0051] Processes according to this aspect of the invention can beapplied to contacts of any size and shape. However, they are especiallyvaluable in the case of contacts having a close pitch, i.e., having apitch or center-to-center distance P below about 250 microns andtypically below about 200 microns. Typically, each contact has awidth-wise dimension w_(p) between about 70 and 100 microns. In the caseof round contacts, the width-wide dimension is simply the diameter ofthe contacts. The length of l_(s) of each strip desirably is greaterthan 1.0 times the dimension l_(p) of the contact pad in the directionparallel to the length of the strip. The upper limit on solder-pastestrip length l_(s) is set by surface tension considerations, thisdistance should not be so great that, upon heating the molten solder atthe tips of the strip coalesces into separate droplets and is not pulledonto the contact pads. Typically, the strip width w_(s) is less than orequal to the widthwise dimension w_(p) of the contact.

[0052] In the arrangement of FIG. 7, the solder paste or other bondingmaterial is applied so as to form blobs 322 on one end of each strip320. The blobs are arranged so that the blobs on alternate strips aredisposed on opposite sides of the row as pads. This provides more solderin a strip having a given length l_(s). Because the blobs are disposedon opposite sides of the row, this arrangement does not materiallyreduce the edge to edge spacing S_(s) between adjacent solder strips. Ina further variant, (FIG. 8A) the solder masses 320 are not in the formof elongated strips. However, each solder mass is applied so that thecenter of area 421 of the solder mass 420 is offset from the rowcenterlines 417. The centers of area 421 of adjacent solder masses 420along the row are offset from the row centerline in opposite directions.Here again, the edge to edge spacing of S_(s) between adjacent soldermasses is greater than that which would be achieved by solder masseshaving sizes shapes and placement conforming to the shapes of thecontact pads 416. Many different shapes of contact pads and soldermasses can be used. For example, square, triangular or other contact padshapes may be employed at a square, triangular, or other solder massshapes. The arrangement of FIG. 7 discussed above also provides thesolder masses with centers of area 321 offset from the centerline of therow on opposite sides thereof.

[0053] As seen in FIG. 8B, a chip or other element may have rows ofcontacts 446 extending in different row directions. Thus, the two rowsof contacts partially shown in FIG. 8A extend in mutually orthogonal rowdirections R₁ and R₂, and approach one another at a corner 447 of thechip. The strips 440 of bonding material may be applied so that thestrips crossing both rows of contacts extend in a common strip directionwhich is oblique to both row directions. For example, the direction ofelongation of strips 440 may be at a 45° angle to both row directions.The use of a common strip direction makes it easier to apply the stripsuniformly. Also, positioning of the strips in this manner avoidsinterference between strips at the corner.

[0054] A connection component 530 (FIGS. 9 and 10) utilized in a processaccording to a further embodiment of the invention includes a dielectricbody 532 having conductive features in the form of leads 544 on a firstor top surface 533. In this embodiment as well, the leads incorporatetips ends 545 releasably attached to the body 530 and terminal ends 546permanently secured to the dielectric body by terminal structures 548.Component 530 also includes temporary attachment elements or securements502. Each temporary securement element includes a “dummy” pad having arelease portion 506 in the center of the pad and a attachment portion oranchor portion 508 extending around the periphery of the pad. Holes 510are provided within body 532 in alignment with the dummy pads, so thatthe release portion 506 is aligned with the hole 510 whereas the anchorportion 508 extends beyond the peripherally of the hole and is engagedwith the dielectric body 532. Each temporary securement 502 alsoincludes a mass of an adhesive 512 disposed atop the release portion 506of the dummy pad.

[0055] Adhesive 512 desirably is adapted to form adhesive bonds to thesurface of a microelectronic element, which will be used with thecomponent. For example, where the microelectronic element is to beunited with a component incorporating passivation layers on its frontsurfaces, the adhesive 512 preferably is adapted to form an adhesivebond to the passivation layer. The adhesive desirably is arranged toform these bonds at or near room temperature such as below about 100° C.and preferably below about 60° C., and under ordinary atmosphericconditions. As further discussed below, the adhesive used in thesecurement 502 need not provide a reliable adhesive bond underconditions encountered in service. Essentially any adhesive which iscapable of forming a bond of sufficient strength to hold themicroelectronic component in position during the steps prior toformation of electrical connections as discussed below, and which doesnot contaminate the assembly can be employed. For example, epoxies,acrylics and even conventional household glue of the type sold under theregistered trademark Elmer's Glue-All can be employed. Where theadhesive tends to cure or “set” after application, the adhesive can beapplied shortly before the component is used.

[0056] In a process according to a further embodiment of the invention,microelectronic elements such as semiconductor chips 510 are united withcomponent 530. The microelectronic elements or chips have rear surfaces512 and front surfaces 514 with contacts 516 (FIG. 10) thereon. Themicroelectronic elements also have a passivation layer 518 coveringthose regions of the front surface 514 not occupied by contacts 516.Here again, the microelectronic elements bear solder masses 520 on theircontact 516. These solder masses may be applied by any of the processesdiscussed above as, for example, by wave soldering as discussed abovewith reference to FIG. 1 or by paste application as discussed above withreference to FIGS. 6-8, or by conventional processes. Themicroelectronic elements are placed onto the connection component 530 ator near room temperature, under normal atmospheric conditions usingconventional pick and place equipment equipped with conventional robotvision systems. Under the control of the robot vision equipment, eachmicroelectronic element is aligned with component 530 so that thecontacts 516 and the solder masses 520 thereon are aligned with theconductive features of the component. However, the temperature at thisstage of the process is too low to activate the electrically conductivebonding material or solder 520 aligned with the conductive features.Instead, each element 510 is held in position temporarily by thetemporary connecting elements 502 and by the adhesive bond 512. Theability to place the microelectronic elements onto component 530 at ornear room temperature and under standard atmospheric conditions greatlyfacilitates placement of the microelectronic elements onto thecomponent. For example, because the polymeric material in dielectricbody 532 is maintained below its glass transition temperature duringthis step, the dielectric material remains dimensionally stable. Also,there is no need to operate the robotic vision and placement systemwithin an environmental chamber. The size of solder masses 520 isgreatly exaggerated in FIG. 9, and in the succeeding figures for clarityof illustration.

[0057] The component 530 with the microelectronic elements thereon isplaced onto a fixture 570 (FIG. 11). The fixture includes a metallicplate 572 having a top surface 574 and a groove 576 extending around acentral region 578 of the top surface. Groove 576 is open to the topsurface. The fixture also has one or more vacuum ports 580 communicatingwith groove 576. One or more additional ports 582 and 584 also may beprovided. Additional ports 582 and 584 communicate with the centralregion 578 of the top surface through internal channels within plate572. Desirably, the fixture, the ancillary equipment (not shown) usedwith the fixture, both include some arrangement for controlling thetemperature of plate 572. For example, plate 572 may be provided with aninternal channel 586 for conducting a temperature control fluid throughthe plate. Other temperature control arrangements which may be usedinclude electrical resistance and/or thermoelectric heaters;thermoelectric cooling devices and other conventional heating or coolingdevices. Desirably, these devices are equipped with appropriate feedbackcontrol systems for maintaining the desired plate temperature. Thefixture are similar to the fixtures described in co-pending, commonlyassigned U.S. patent application Ser. No. 09/174,074 filed Oct. 16,1998, the disclosure of which is hereby incorporated by referenceherein.

[0058] After the microelectronic elements 510 and component 530 are inplace on fixture 570 a deformable barrier in the form of a flexible film590 is disposed over the rear surfaces 512 of the microelectronicelements and over fixture 570. Film 590 may or may not form a part ofthe completed microelectronic assembly as discussed below. In thoseapplications where the film does not form a part of the completeddevice, the film may be formed from essentially any material which iscapable of withstanding the temperatures and pressures encountered inthe assembly operation. For example, films formed from polyamide, fluoropolymers and the like may be employed. The firm desirably is transparentor translucent to radiant energy used in the process. However, even if apolymer has appreciable absorptivity for infrared radiation or otherradiant energy used in the process, a relatively thin film of such apolymer may have a relatively small absorption and hence may be usefulin the process.

[0059] Film 590 may be clamped against the top surface of the fixture bya clamping ring 592 (FIG. 12) disposed outside of channel 576. The spacebetween the film 590 and the fixture preferably is flushed with a gasthat is substantially non-oxidizing to the bonding material and otherelements during the bonding process as, for example, nitrogen, carbondioxide, argon or helium. After gas flushing, gas is withdrawn throughport 580, and may also be withdrawn through one or more of auxiliaryports 582 and 584. These ports may be connected to a conventional vacuumpump for this purpose. This brings the working space 594 between film590 and component 530 to a subatmospheric total pressure. Also, at thistime the partial pressure of oxygen within working space 594 issubstantially lower than the partial pressure of oxygen in normalatmospheric air, i.e., below about 160 Torr. This low partial pressureof oxygen is due to two factors: the low total gas pressure and the lowproportion of oxygen in the residual gas present in working space 594.

[0060] The surrounding atmosphere bears on the exterior surface of thebarrier or film 590. Because the atmospheric pressure is higher than thepressure within working space 594, there is a pressure differentialacross the barrier or film 590 urging the barrier or firm towardscomponent 530. The barrier or film thus urges the microelectronicelements 510 towards the component and hence urges the electricallyconductive features, such as the contact 516 on the microelectronicelements (FIG. 10) towards the corresponding leads or other conductivefeatures 544 on the component 530. The barrier or film may distort asshown in FIG. 13.

[0061] Although the film applies substantial downward forces to themicroelectronic elements, it does not apply substantial net forces toany of the semiconductor chips or microelectronic elements 510 inhorizontal directions, parallel to the top surface 533 of component 530.Therefore, the individual semiconductor chips remain substantially inthe positions where they are placed during the initial placement step(FIG. 9). The downward forces, however, assure good engagement betweenthe conductive features on the microelectronic elements 510 andcomponent 530, even where the microelectronic elements are slightlydifferent thicknesses and/or where the microelectronic elements, thecomponent or both are slightly out of planarity. Thus, the bondingmaterial masses 520 on the conductive features 516 of elements 530 areforced into engagement with the tip ends 545 of leads 544. The temporarysecurements 502, and particularly the adhesive masses 512, can deformslightly in compression so as to allow some movement of chips ormicroelectronic elements 510 toward component 530.

[0062] During these stages of the process, the plate 572 constitutingfixture 570 is maintained at a temperature below the glass transitiontemperature of the polymer constituting the body of component 530.Desirably, the fixture is maintained at a temperature slightly below thesolidus temperature of the solder constituting the solder masses 520.Under these conditions, oxides and other compounds found on the surfacesof the solder masses and on the surfaces of the conductive featuresdecompose, leaving clean metal surfaces in a manner similar to thatdiscussed above. Depending on the composition of the adhesive 512 insecurements 502, the temperature used in this operation may cause theadhesive to degrade. However, the secure engagement provided by the filmwill hold the chips in place.

[0063] In the next stage of the process, while the total pressure withinthe working space 594 is maintained below atmospheric pressure, andwhile the partial pressure of oxygen is maintained below the partialpressure of oxygen in normal room air, the engaged conductive features516 and 544 are heated momentarily by applying radiant energy to therear surfaces of the microelectronic elements 510, through a barrier orfilm 590 as schematically depicted in FIG. 14. The radiant energytypically heats the conductive features by absorption of the radiantenergy in the microelectronic element and conductive heat transfer fromthe microelectronic element to the contact pads 516, solder masses 520and leads 544. Also, some of the radiant energy may pass through themicroelectronic element and may be absorbed by component 530 adjacent totop surface 533 thereof. Here again however, the mean temperature ofbody 532 desirably is maintained below the glass transition temperatureof the polymer constituting the body. As discussed above with referenceto FIGS. 1-4, this helps to maintain the dimensional stability of thepolymeric body. Here again, the intimate contact between the dielectricbody 532 and fixture plate 572 promotes good heat transfer between thedielectric body and the fixture and helps to maintain the temperature ofthe dielectric body close to that of the fixture plate. Film 590continues to urge the microelectronic elements or chips 510 downwardly,towards component 530 and hence continues to urge the conductivefeatures into engagement with one another at this stage of the process.

[0064] The radiant energy application conditions can be similar to thosediscussed above with reference to FIGS. 1-4. However, the radiant energyapplication step of FIG. 14 can be conducted while normal atmosphericconditions prevail outside of working space 594. Stated another way, thefilm and fixture cooperatively form a small vacuum chamber enclosing theparts to be treated. Accordingly, there is no need to hold the fixtureinside a separate controlled-environment chamber during the radiantenergy application step. This greatly simplifies the process andfacilitates the handling of numerous parts on a mass-production basis.

[0065] The applied radiant energy raises the solder above its solidustemperature and desirably above its liquidus temperature and causesformation of metallurgical bonds between conductive features on elements510 and component 530. Thus, the contact pads 516 are bonded to the tipends 545 of leads 544 by molten solder masses 520. During this step, thefilm or barrier continues to urge the microelectronic elements andcomponent towards one another. Adhesive masses 512 (FIG. 10) may deformslightly or degrade during this process. These factors facilitateengagement of the molten solder masses with the conductive elements ofcomponent 530, i.e., with lead tip ends 545.

[0066] Upon termination of the radiant energy application, theconductive features, microelectronic element and component 530 coolrapidly. The presence of normal, atmospheric air on the film surfaceoverlying the rear surfaces of microelectronic elements 510 facilitatescooling of these elements by conduction through film 590 and conventioncooling into the surrounding atmosphere. Component 530 is cooledeffectively by heat transfer to plate 572.

[0067] After cooling, metallurgical bonds exist between contact pad 516and lead tip ends 545. In the next stage of the process, a flowable,curable material such as a polymeric encapsulant forming composition isinjected under pressure between component 530 and film 590 and hencebetween chips or microelectronic elements 510 and component 530. Thevacuum present in working space 594 facilitates injection of the curablecomposition and assures that the curable composition can penetrate intoall spaces present in the assembly as, for example, around leads 544 asschematically illustrated in FIG. 16. For example, the curablecomposition may be injected through one of the auxiliary ports 584. Oneor more ports communicating with working space 594 may be left connectedto a vacuum source during the beginning portion of the injection cycleto facilitate removal of gases evolved from the encapsulant and towithdraw any remaining gas from the working space. However, these portsare desirably are closed before completion of the encapsulant injectionso that the pressure within the flowable composition increases to anabsolute pressure above normal atmospheric pressure, i.e., to a positivegauge pressure. Under these conditions, the encapsulant urges the filmand the microelectronic elements 510 upwardly away from component 530and causes the microelectronic elements 510 to move upwardly, away fromcomponent 530. If not previously broken, the temporary connections 502break during this process. As best seen in FIG. 16, the release portions506 of the dummy pads break away from the anchor portions 508, therebyallowing the chip or microelectronic element 510 to move upwardly awayfrom component 530. As the microelectronic element moves upwardly awayfrom component 530, the tips ends 545 of leads 544 are also movedupwardly, away from the body of component 530 thereby deforming theleads 544 towards a vertically extensive disposition. The size andvertical extent of leads 544 is greatly exaggerated in FIG. 15 forclarity of illustrations. In fact, a vertical displacement of about 50to about 200 microns typically suffices. A platen 596 (FIG. 15) may beprovided in engagement with the exterior surface of film or barrier 590so as to control the vertical movement of the film and of the chips ormicroelectronic elements 510. In a further variant, the platen may beprovided with vacuum passages (not shown) for engaging the film andhence holding the film and microelectronic elements against the platen.The platen and hence the microelectronic elements 510 may be movedrelative to fixture 572 by an external movement device (not shown) so asto move the microelectronic element relative to component 530independently of curable material injection. In a further variant,movement of the film 590 and microelectronic element may be produced byinjecting a gas or inert liquid into working space 594. The resultingparts may be encapsulated at a later stage or left unencapsulated.

[0068] During injection and curing of the flowable encapsulant, film 590protects the rear surfaces 512 of the chips, so that those surfacesremain free of encapsulant after curing. After injection and curing ofthe encapsulant, the parts are removed to form the fixture and thecomponent 530 is severed so as to form individual assemblies, eachincluding one or more of the microelectronic elements and portions ofthe component associated herewith and connected thereto by leads 544.Portions of film 590 remaining on the rear surface of each chip may bepeeled away, to expose a clean surface 512 for connection to a heatsink. Alternatively, the film 590 may be peeled away before severing.

[0069] As shown in FIG. 17, the flexible barrier or film 690 may besealed to component 630 directly or else by means of a sealing ring orframe 691 extending around the periphery of the component. Also, as seenin FIG. 17, the deformable barrier or film 690 need not be continuousbut may have openings 693 aligned with the microelectronic elements 610,provided that the film 690 forms a seal to the microelectronic rearsurface 612 at the periphery of each opening 693. As will be appreciatedfrom FIG. 17, the film 690 forms a top sealing film whereas thecomponent 630 itself forms a bottom sealing film. These films,cooperatively with frame 691 enclose the working space 694. Here again,a subatmospheric total pressure within working space 694 causes thesurrounding atmospheric pressure to bear on the films and urge theconductive features 644 of the components 630 together with theconductive features 616 of the microelectronic components. As discussedin detail in co-pending, commonly U.S. patent application Ser. No.09/067,698 filed Apr. 28, 1998, the disclosure of which is herebyincorporated by reference herein, a frame for forming a sealed unit usedin semiconductor processing may be formed from a relatively inexpensivematerial such as a polymer so as to provide a disposable frame. As alsodisclosed in the '698 application, vacuum can be applied within theworking space enclosed by such a frame and membrane assembly, and afluid such as a curable encapsulant composition may be injected intosuch a space using needles penetrating a film. Alternatively oradditionally, the frame may incorporate ports and connections. Forexample, where the frame 691 is produced by a process such as injectionmolding, passageways 680 may be molded into the frame so as to provideports for gas flushing, vacuum application and encapsulant injection.These ports may be used in the same manner as the ports discussed above.As described in greater detail in the '682 application, the use ofdisposable frames essentially eliminates the task of cleaning fixturesso as to remove cured encapsulant. Typically, after the various steps ofthe process has been performed, the frame is cut into pieces when thecomponents 630 is severed so as to form individual units as describedabove. The pieces of frame are removed and discarded.

[0070] As seen in FIG. 18, the microelectronic element may include awafer 710, whereas the component 730 may incorporate a flexible,dielectric film body 732. Film 732 may be sealed to the component 710,or preferably, to a fixture 770 similar to the fixtures discussed abovewith reference to FIGS. 9-14. In this instance, the flexible body 732 ofcomponent 730 itself serves as the deformable barrier. When a vacuum isapplied within working space 794, the pressure of the surroundingatmosphere urges the barrier downwardly, towards fixture 770 and thusurges the conductive features 744 and components 730 into engagementwith the conductive features or contact pads 716 on wafer 710.

[0071] In the processes discussed above, the ability to conduct variousprocess steps such as momentary heating to activate the solder orbonding material under normal, atmospheric conditions greatlyfacilitates mass production. Further, the deformable film reliably urgesthe conductive elements towards one another. Numerous variations andcombinations of the features discussed above can be employed. Forexample, in the process of FIGS. 9-14, the conductive features oncomponent 530 include elongated leads and are subsequently bent bymoving the microelectronic elements away from component 530. Asdescribed in greater detail in U.S. Pat. No. 5,518,964, controlleddeformation of the leads towards vertically extensive dispositionprovides numerous advantages. However the connection process accordingto this aspect of the invention can be applied to any conductivefeatures. For example, the conductive features 744 and elements 730 mayinclude simple contacts which cannot be deformed, and these contacts maybe bonded to the contact pads 716 of a wafer. Also, bonding materialsother than solders may be employed as, for example, eutectic bondingalloys, diffusion bonding alloys and thermoplastic compositionsincluding conductive particles. The bonding material may be carried onthe conductive elements of the component rather than on the conductiveelements of the chip or other microelectronic element.

[0072] In a further variant, the temporary connecting elements 502discussed above with reference to FIGS. 9-14 need not incorporate abreakable dummy pad as discussed above. Rather, these bonding elementsmay include a mass of adhesive 512 which can peel away from thecomponent 530 or the microelectronic element 510 when these elements areforced away from one another. Alternatively or additionally, theadhesive in masses 512 may be arranged to melt or decompose during themomentary heating step used to melt solder masses 520 or earlier, whenthe components are heated to remove oxides prior to bonding.Alternatively or additionally, the adhesive masses may be arranged tomelt or dissolve in the encapsulant when the encapsulant is injected.

[0073] In a further variant, the temporary connecting elements orsecurements may include masses of an adhesive 812 (FIG. 19) which areinitially carried on the chips 810 or other microelectronic elements andwhich are adapted to bond with the component 530 when themicroelectronic elements are assembled with the component. The componentmay optionally have pads 807 adapted for bonding with the adhesivemasses. These pads may optionally be breathable pads as discussed above.The adhesive masses and pads are positioned so that when the conductivefeatures 816 of the chips are aligned with the conductive features 844of the components, the adhesive masses are aligned with the pads.Alternatively, the adhesive masses 812 may be adapted to bond with thedielectric material of the component 830, and the pads 807 may beomitted. The adhesive masses may be applied to the chips before or afterthe chips are severed from a wafer. In other respects, the process issimilar to those described above.

[0074] In a further variant, shown in FIG. 20, the temporary securementsmay include adhesive masses 912 disposed over a first set 944 a of theleads 944, so that the adhesive masses 912 extend between the leads ofthe first set and the microelectronic element 910. Thus, the leads 944 aof the first set serve as pads or attachment points for the adhesivemasses. Here again, the adhesive or temporary securement holds themicroelectronic element in place relative to the component while theconductive elements or contacts 916 are bonded to the tip ends 945 ofthe leads by bonding material 920. The adhesive peels away from themicroelectronic element, from the leads 944a of the first set, or bothwhen the microelectronic element is moved away from the component in tobend the leads as discussed above. Alternatively or additionally, theadhesive may be degraded or dissolved as discussed above. Preferably,the first set of leads 944 a includes less than all of leads 944. Theleads 944 a of the first set may be non-functional leads which do notprovide electrical connections in the completed assembly. Therefore, anydamage to these leads which may be caused by the forces which may beapplied to these leads by the adhesive during movement will not disablethe assembly. Alternatively, the leads of the first set may be robustenough to withstand these forces. As disclosed in U.S. Pat. No.5,976,913, the disclosure of which is hereby incorporated by referenceherein, some of the leads in a particular assembly may be used asrestraining straps to constrain the movement of the microelectronicelement and component. These leads, referred to as “restraining straps”,can be shorter and stronger than the other leads in the assembly. Theleads of the first set may constitute restraining straps.

[0075] As shown in FIG. 21, the adhesive may overlie the tip ends 1045 aof a first set 1044 a of leads 1044. The tip ends of all of leads 1044,including first set 1044 a, are releasably attached to the body ofcomponent 1030, as by small, breakable elements 1031 integral with theleads or the component body, or by a weak bond between the tip ends ofthe leads and the body, whereas the fixed or terminal ends 1046 arepermanently fixed to the body. When the chip or microelectronic element1010 is assembled to the component 1030, the adhesive bonds themicroelectronic element to the tip ends 1045 a of first-set leads 1044a, so that the adhesive and the releasable attachment between tip ends1045 a and the component form temporary securements between holding themicroelectronic element in place on the component during bonding of theconductive features of the microelectronic element and component byconductive bonding material 1020. When the microelectronic element andcomponent are moved away from one another, the tip ends of first-setleads 1045 a, as well as the tip ends of the other leads, are peeledaway from the component body, thereby breaking the temporary securementby release of the attachment between tip ends 1045 a and the componentbody. In this embodiment, the adhesive need not be degraded or brokenduring the process. The leads 1044 a of the first set adhesive may benon-functional leads, and may be restraining straps as discussed above.Alternatively, conductive bonding material may be provided for theseleads as well, on portions of their tip ends not covered by adhesive, toestablish electrical connection to the microelectronic element. In yetanother variant, the adhesive used in the temporary securements may beelectrically conductive, and conductive features (not shown) may beprovided on the microelectronic element in registration with the tipends 1045 a of the leads in the first set. Thus, the leads of the firstset may provide additional electrical connections.

[0076] As these and other variations and combinations of the featuresdiscussed above can be utilized without departing from the presentinvention as defined by the claims, the foregoing description of thepreferred embodiments should be taken by way of illustration rather thanby way of limitation of the claimed invention.

What is claimed is:
 1. A unit including one or more semiconductor chips,each such chip having a front surface with a plurality of contactssurrounded by a passivated surface not wettable by bonding material,said contacts having masses of bonding material thereon, said masseshaving height less than the diameter of said contacts.
 2. A unit asclaimed in claim 1 wherein said contacts are less than about 100 μm indiameter
 3. A unit as claimed in claim 1 wherein said bonding materialis a solder.